General growing instructions for flowers, herbs & vegetable plants
General guide to seed sowing
Most plants germinate successfully under very similar conditions and this guide will therefore give you the right tools, for growing most native plants of local origin. Exotic plants from different climatic zones such as cacti and palm trees however, may require special conditions or treatment to start germination and it is always recommended to do a little research before starting the sowing process.
Three factors are key for germinating plant seeds: water, temperature and light.
Generally speaking, contact with water will start the germination process in the case of most dried seeds. The seed needs to absorb enough water to swell up and to eventually burst open. At the same time, it is essential not to administer too much water either.
The germination process requires a lot of energy, which is generated with the help of fair supplies of oxygen. If the soil is too dense, too wet or even frozen, oxygen will not reach through to the seeds and the germination process may be disrupted.
In terms of providing the right temperature for germinating seeds, the temperature, which best simulates the average spring temperature of the climatic zone, of the plant’s country of origin, has often promoted the best results.
As a rough guide, most local varieties germinate well in temperatures around +8-18°C, while plants from warmer regions may require around +18-26°C, to reach germination. If latter is the case, successful results may only be achieved in a heated greenhouse, where temperatures can be kept and managed at a consistent level. High temperatures (above +30°C) however, may yet again impede and disrupt the germination process. As to lighting conditions, in particular small seeds will need substantial exposure to light. Due to their size, these seeds only have small nutrient reserves and depending on the natural light levels of their growing period, additional support for germination may be needed, in the form of a special plant light.
An additional factor to be taken into consideration is the sowing depths when germinating seeds. The sowing depths depends on the size of the seed and as a rule of thumb, it is worth remembering, that the coverage with soil should amount to the same measurement, as the diameter of the seed. As a consequence, most seeds are purely spread out on top of the seedbed or merely covered by a thin dusting of well-sifted soil. Rarely will you find recommended sowing depth beyond 0,5-2cm.
It will come as no surprise that choosing the right soil will also impact the successful growth of your plants. For germinating seeds, special potting soil or potting compost is readily available, which often consists of a mix of sand, peat or peat substitutes, such as coco peat, which is a substrate or perlite. Ideally potting soil needs to be cleaned of germs or insects, which could have plant-damaging impact and comes finely ground to provide good contact between seed and soil. Fine potting soil will allow oxygen to reach the seed and to store water appropriately. However, potting soils often hardly contain any fertilisers and are not too nutrient-rich, to avoid that plants grow too quickly before they developed a self-providing, robust root system.
Light germination/ dark germination/ cold stratification / seed soaking/ seed scarification
Some plant varieties have adapted and specialized to a level, which has also effected their germination requirements. Therefore, their seeds may only germinate under strict set conditions:
- Some seeds really only germinate in darkness and therefore will need coverage with potting soil twice their own thickness.
- Other species only germinate exposed to light and need spreading out on top of the potting soil and careful watering with a water spray bottle.
A third category of plant seeds will only germinate after their dormancy period has been interrupted by exposure to cold temperatures before being sown. This is a smart mechanism created by nature to guarantee that seeds will only start to germinate after the passing of the winter period in springtime. The seeds of this category contain substances which supress the germination process, until their quantities have been reduced sufficiently in response to the cold. Consequently the sowing period for these plants tends to be between November and January. However, the seeds still need to be kept moist and warm (ca. +20°C) for 2-4 weeks, spread out in pots or seed raising trays before being moved outdoors for a couple of weeks at least, exposed to temperatures around -4 and +4°C consistently. In spring, when temperatures rise naturally, the germination process will start automatically, often taking a couple of weeks. It is of course also possible to recreate this ‘low temperature stimulus’ artificially, by keeping the seed cultures in their pots in an old fridge if available. In this case it is essential though, to check the temperature regularly.
Far more practical is another handy procedure. For this, the seeds will be mixed with a small quantity of slightly damp sand, perlite, vermiculite or a mix of all three and put into an airtight container or zipper bag, which again needs be kept in the fridge (and never in the freezer) for approx. 8 weeks at +5°C. It’s advisable to keep an eye on the seeds regularly as they sometimes may start germination early. This artificial treatment of seeds with low temperatures is also called cold stratification of seeds.
Many plant seeds, often deriving from habitats with seasonal variations in temperatures or pronounced climatic differences, such as seasonal drought-, or rainfall periods, undergo a dormancy period. It can be regarded as a natural safety mechanism, to supress the germination process, when the conditions for successful plant growth are compromised. Natural causes for the dormancy period usually are physical barriers that block the germination process, such as firm seed coat strength or permeability, unfinished embryo growth or high levels of germination supressing substances stored inside the seed. Usually, natural processes, such as cold temperatures during wintertime, break down these barriers blocking the germination process. Of course there are also methods and tricks how to break the dormancy period artificially to kick off the germination process, however, in this case, it is absolutely necessary to keep conditions consistent (temperature, light, water, humidity), in order to avoid the seeds to shift into a second dormancy period, which is much more difficult to break.
If the germination process is blocked by germination inhibitors stored inside the seed, pre-soaking them in hot water, milk or herbal extracts for a short while can reduce these substances, speed up germination and as in a few cases, disinfect them against fungi and diseases. For example, liquid extracts from plants such as: valerian, chamomile, garlic, campion or horsetail/scouring rush have proven very successful.
Seed scarification is a mechanical method, to break through seed coat dormancy, which is caused by the presence of hard seed covering. The physical barrier of the seed needs to be reduced or weakened in order to allow the seed to absorb water. Depending on the size of the seeds, they carefully can be rubbed down lightly, with sandpaper or sand. Is the seed coat particularly thick, it might be required to use a knife, to scrape off and thin down its surface. Afterwards allow the treated seeds to soak in water for about 24 hours.
Indoor sowing, cold frame-, or greenhouse cultivation
It is recommended to start plant seeds as a pre-culture indoors or in a greenhouse if they take long to reach maturity or need higher temperatures to germinate. Pre-cultures can be started from January to April depending on the plant species.
- It might be necessary to treat larger seeds before starting the sowing process (please see seed pre-treatments above: seed soaking/ seed scarification)
- If you have used your seed trays/ propagators/ planters in a previous season, please make sure that you give them a thorough clean first, before filling them with potting soil/ compost (ideally peat free and patted down lightly). Whichever container you may find useful for starting your seeds, always make sure that they have enough holes for draining off excess water, to avoid waterlogging and promote air circulation.
- If your plant seeds require light to germinate successfully, you can simply spread them out on top of the soil and pat them down lightly, without covering them up. Depending on the size of your seeds, which need to germinate in darkness, you may choose to use a widger, dibber or suitable prong, matching the size of your seed, to prepare accurate holes, to imbed them in carefully. Cover them lightly with more soil and pat down the surface. If you are using bigger pots or boxes, a tamper tool might be helpful to pat down the soil evenly and firmly enough. Avoid sowing too many seeds in a pot. As a guide, a pot with a diameter of about 9cm can take 3-5seeds. Seedlings will develop stronger with more space and it will be also easier to transplant them later.
- Highly recommended: Use plant labels and keep track of your seedlings. Particularly the first small developing leaves often look very indistinct and it may be very difficult to differentiate your seedlings from each other otherwise.
- Your seed pre-cultures are now ready to take their place in a mini greenhouse or on top of a dripping mat. They need to be watered now to start the germination process, however, it is advisable to use a bottle spray on seeds, which need light for germination, to avoid silting up. Alternatively, you could position the pots on a tray of water and let the soil soak it up from below. The potting soil needs to be moistened well and evenly but not be too wet, to allow for good oxygen circulation.
- Close the cover of your greenhouse, seed tray or propagator and put clear plastic bags over your individual pots to achieve the same effect. Choose a light and consistently warm location for your pre-cultures. Some warmth loving plants may require a spot nearby a radiator or the use of a special heatable mat for plants, which usually has a thermostat to help with controlling the correct temperature automatically. It is very important to keep the plant cultures covered to keep in the moisture and to maintain good humidity levels. Often further watering may not be required until the first shoots appear, however the soil may never dry out completely.
- Once the first seed leaves (also called cotyledons) develop, the temperature may be lowered slightly. Sufficient light exposure and good supplies with oxygen are essential for this stage now. Ideally, emerging seedlings remain short and compact at the beginning to gain strength and grow into robust seedlings ready for pricking out. If your seedlings grow in height too quickly, growing conditions may need to be adjusted accordingly.
- Your seedling is ready for pricking out once the first pair of true leaves has fully formed above the earlier seed leaves (approx. 3-6 weeks after sowing the seeds). Carefully loosen the soil surrounding the fragile seedlings with a widger or a tapered ice-lolly stick and scoop the seedling out from below. It is often unavoidable to cause some minor damage to the delicate roots. In moderation, this will actually stimulate healthy root growth development. Your seedling is ready to be transplanted into a larger container or pot and also requires more nutrients for the following development stage now.
- Before finally transplanting the young plants into patches or large pots outdoors, they need to be hardened off first. Once outdoor temperatures are stable, position your plant pots or trays outside, in a sheltered position, without too strong direct sunlight exposure, which they are not used to yet. The idea is to acclimatize the young plants to the harsher conditions outdoors and it may be advisable to move them inside over night if frosts may still occur, or to provide extra protection by using a cold frame or protective fleece.
Equipment needed for indoor sowing
- Sufficient supplies of sowing-, growing-, or module trays with covers, seedling pots or/and plant saucers, usually available in a wide range of materials, such as plastic, clay or biodegradable cellulose.
- Potting soil/potting compost or coir pellets/propagation plugs.
- If your heart is set on growing plants, which require higher germination temperatures you may need a windowsill greenhouse, electrical propagator or a heating mat. Other useful/essential equipment includes: a thermometer, a widger or dibber, a wooden tamper, a potting sieve, a water spray or watering can and plenty plant label sticks.
Direct sowing outdoors
Many flower and vegetable plants are well suited for direct sowing outdoors. However, it is strongly advisable to wait until the passing of the night-frost period, which may last till mid May in some regions. Meanwhile take your time to prepare the seedbed: loosen the soil and sieve it through where required, to bring it to a fine, crumbly texture. Remove weeds, stones and old root nodes. It is common practice for sowing outdoors, to either spread out the seeds randomly or to grow them in lines. If the latter is desired, you can easily prepare trenches measuring the required sowing depth and simply place your seeds into them at the recommended sowing distance. Cover the seeds well with soil, press it down firmly and water the patch gently with a soft water-jet only. For further protection and care you may choose to cover your seedbed with a damp fleece. Always keep your seed culture evenly and sufficiently watered and enjoy the wait till the first shoots appear. Once your seedlings have developed into young plants, they may need to be spread out or transplanted to their recommended planting distance.
Equipment needed for outdoor sowing
Pick your favourite rake, hoe, spade, shovel and/ or claw cultivator for loosening and flattening the seedbed, for creating sowing trenches and for covering the seeds with soil at the end. Furthermore it might be useful to have a planting ruler, planting tape and a dibblet on hand. Also needed for regular watering is a watering can or garden hose, ideally with a soft water-jet.